1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine that is provided with air-fuel ratio sensors at upstream and downstream sides of a catalyst. The air-fuel ratio control device performs air-fuel ratio control based on the output of air-fuel ratio sensors at the upstream and downstream side.
2. Description of the Related Art
Normally, a three-way catalyst is provided in an exhaust passage of an internal combustion engine to simultaneously purify HC, CO, and NOx contained within the exhaust gas. With this catalyst, the purification rate of each one of HC, CO and NOx is increased in the vicinity of the stoichiometric air-fuel ratio. Accordingly, normally, an air-fuel ratio sensor is provided at the upstream side of the catalyst and the air-fuel ratio controlled to be in the vicinity of the stoichiometric air-fuel ratio. Here, a structure will be explained in which oxygen concentration sensors are used as air-fuel ratio sensors for detecting the concentration of specific components contained in the exhaust gas. Hereinafter, the term “oxygen concentration sensor” will be referred to as “O2 sensor”.
The upstream O2 sensor provided at the upstream side of the catalyst is positioned at a location in the exhaust system that is as close as possible to the combustion chamber, namely, in the merging area of the exhaust manifold that is upstream of the catalyst. However, the upstream O2 sensor is exposed to high exhaust temperatures and poisoned by various kinds of toxic substance, and thus the output characteristics of the O2 sensor vary substantially. In order to compensate for this variation in characteristics, dual O2 sensor systems have already been proposed in which a downstream O2 sensor is provided at the downstream side of the catalyst. In these dual O2 sensor systems, in addition to the upstream O2 sensor being used to perform a first air-fuel ratio feedback control, the downstream O2 sensor is used to perform a second air-fuel ratio feedback control. Examples of such systems are disclosed in JP-A-63-195351 and JP-A-06-42387.
Although the response speed of the downstream O2 sensor is comparatively slow compared to that of the upstream O2 sensor, the downstream O2 sensor has the following advantages. The impact of heat on the downstream O2 sensor is limited since the exhaust temperature is low at the downstream side of the catalytic converter, and poisoning is also low since the catalyst traps the various kinds of toxic substance. Accordingly, variation in the output characteristics of the O2 sensor is small. In addition, at the downstream side of the catalyst, the exhaust gas is mixed more thoroughly and thus the purification state of the catalyst positioned at the upstream side can be detected more stably.
In the dual O2 sensor system, the output of the downstream O2 sensor is controlled to a target value, thus allowing the variation in the output characteristics of the upstream O2 sensor to be compensated for by the downstream O2 sensor. Accordingly, the purification state of the catalyst can be favourably maintained.
The catalyst has oxygen storage capacity in order to compensate for temporary variations in the upstream air-fuel ratio from the stoichiometric air-fuel ratio. When the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the catalyst absorbs and stores oxygen within the exhaust gas, whereas when the air-fuel ratio is rich, oxygen stored in the catalyst is released. In this manner, the catalyst has an effect like filter processing, and variations in the upstream side air-fuel ratio are filter processed within the catalyst to generate the air-fuel ratio at the catalyst downstream side.
The oxygen storage capacity of the catalyst is determined by the amount of substance with oxygen storage capacity that is included in the catalyst when it is made. Further, the catalyst of the catalytic converter is exposed to high temperature exhaust gas. Thus, in order that functioning of the catalyst does not deteriorate suddenly under the normal expected usage conditions of the vehicle, the catalyst is designed to be heat resistant. However, there may be occasions when the exhaust gas temperature becomes abnormally high during use due to a cause like misfire. In this case, the oxygen storage capacity of the catalyst drops substantially. Moreover, even under normal usage conditions, if the vehicle's mileage reaches tens of thousands of miles, the oxygen storage capacity of the catalyst will gradually reduce due to age deterioration. Thus, during the initial period after manufacture, the filter action of the O2 storage capacity of the catalyst is large, and the output of the downstream O2 sensor is comparatively stable. However, as the catalyst deteriorates, the filter action also reduces, and thus variation in the air-fuel ratio of the upstream side is transmitted to the downstream side causing the stability of the downstream O2 sensor output to worsen.
In the dual O2 sensor system, the output of the downstream O2 sensor is utilized to correct the air-fuel ratio control using the upstream O2 sensor. However, in the case that the stability of the downstream O2 sensor output has worsened due to catalyst deterioration, the stability of the air-fuel ratio control using the upstream O2 sensor is also impaired. To address this difficulty, a structure has been proposed, such as that disclosed in JP-A-06-50204, in which the output of the downstream O2 sensor is filter processed. Following filter processing, the output of the downstream O2 sensor is used to correct the air-fuel ratio control using the upstream O2 sensor. The time constant of the filter processing is set such that variation in the output of the downstream O2 sensor following catalyst deterioration can be compensated for. Accordingly, even following catalyst deterioration, the stability of air-fuel ratio control does not change.
However, there are times when the upstream air-fuel ratio is made lean or rich such as in fuel cut control, rich control at times of high load, or lean control to improve fuel consumption. At such times, the amount of oxygen in the catalyst reaches the upper/lower limit of the oxygen storage capacity and the atmosphere of the catalyst cannot be maintained at the stoichiometric air-fuel ratio, whereby purification capability drops substantially. Thus, after lean control or rich control is completed, the atmosphere of the catalysts needs to be returned to the stoichiometric air-fuel ratio as rapidly as possible in order to restore purification capability. However, in known air-fuel ratio control devices in which the air-fuel ratio control is performed using the filter processed output of the downstream O2 sensor, there is a delay in detecting the purification state of the catalyst, which leads to a delay in restoring purification capability.